Pulmonary hypertension (PHTN) is common in diseases characterized by chronic hypoxia (CH) (i.e. COPD, IPF) and occurs in 15-40% of patients with sleep apnea. Intermittent hypoxia (IH) mimicking the hypoxia-reoxygenation cycles of sleep apnea causes systemic hypertension and altered regulation of systemic vascular tone. However, the effect of intermittent hypoxia on the pulmonary circulation is unknown. Recently, patients with sleep apnea-induced PHTN were found to have exaggerated hypoxic pulmonary vasoconstriction. Unlike in chronic hypoxia, hypoxia in sleep apnea is not continuous, thus the mechanisms causing sleep apnea-induced PHTN are likely different from chronic hypoxia-induced PHTN. We therefore hypothesize that intermittent hypoxia leads to pulmonary hypertension by differential expression of genes important in regulating pulmonary vascular tone. Specifically, we hypothesize that oxidant stress in IH increases NOS and decreases SOD leading to PHTN through increased formation of peroxynitrite thus decreasing NO available for cellular effects such as attenuating vasoconstriction and mediating vasodilation. We further hypothesize that IH activates redox sensitive transcription factors leading to differential lung gone expression compared to CH. We will present data showing IH-induced PHTN in both rats and mice. We also will present data showing differential expression of NOS (nitric oxide synthase) and SOD (superoxide dismutase) in the lung following IH compared to CH, which may contribute to IH-induced PHTN through increased oxidant stress and decreased NO activity. This proposal will address the questions: 1) does repetitive hypoxia-reoxygenation causes pulmonary hypertension, 2) that despite increased NOS, NO appears to be insufficient to prevent IH-induced PHTN, 3) decreased SOD may contribute to IH-induced PHTN by increasing oxidant stress and formation of peroxynitrite, and 4) does IH leads to differential gene expression through activation of specific signaling pathways compared to CH. We will correlate physiologic measures of PHTN and pulmonary vascular tone with expression and activity of NOS and SOD, measurements of oxidant stress and NO, and activation of specific signaling pathways leading to altered gone expression in IH. This proposal, for the first time, will identify the consequences of IH in the pulmonary circulation. Understanding mechanisms contributing to the development of PHTN in IH may lead improved cardiovascular morbidity and mortality in this common disease.